We have previously covered horrific parasites like Cymothoa exigua and the Cordyceps fungus (links below). Here's another terrifying parasite to add to the list.
Nematomorphs, also known as hairworms, are parasites that mature within their terrestrial hosts. Once the host has been infected, the hairworm will increase greatly in size. The mature worm will be much longer than the host itself and it will fill most of the “host cavity with the exception of the head and legs.” However, once they are sexually mature, the hairworms are only able to reproduce in an aquatic environment. In order to ensure that they will reach the water, some species of nematomorphs have been known to manipulate the behaviour of their hosts.
Infected insects, like grasshoppers, have been known to ‘commit suicide’ by jumping into the water, whereupon the adult hairworm will emerge from its host. Typically, the process has two phases. The host will initially wander into unusual habitats until a water body is reached. After that, the host will jump into the water. Clearly, the grasshopper is not doing this of its own free will. A study has discovered that the gene of the parasite is being expressed as behaviour in the host. The hairworm produces mimetic proteins similar to those already present in the nervous system of the host. By way of these proteins, the hairworm is able to control the hosts movements, ensuring that it is able to reproduce.
In the words of Shelley Adamo, expert in insect behavioural physiology, "It's a very novel study, because there are very, very few papers on how behaviour actually changes."